Increasing awareness of functional heterogeneity among macrophages in their roles as accessory cells in antibody formation and in the regulation of tumor cell growth has led us to an attempt to obtain subpopulations of restricted biological activities. This work has led to the discovery of the "A" cell which, according to accepted criteria, can neither be classified as a macrophage nor a lymphocyte. Its enrichment in that fraction of macrophages (light density) which is particularly active in immunological functions makes it mandatory to undertake the studies proposed here which are aimed at (a) the characterization of the cell with respect to external and internal markers, (b) its isolation, and (c) functional studies of isolated and of reconstituted populations.